In the field of temperature measurement using optical pyrometers, infrared spectrometers, or the like, there is a need for standards and calibration devices which possess emissive properties that approach those of an ideal blackbody, also called a blackbody radiator. For the ideal blackbody, the total radiant emissivity varies directly with the fourth power of the absolute temperature, i.e., EQU M.sup.b (T)=.sigma.T.sup.4
where .sigma. is constant. Materials can be characterized by a total emissivity .epsilon.(T) relating the total radiant emissivity M(T) (total power emitted per unit area) to that of a blackbody M.sup.b (T) at the same temperature by EQU M(T)=.epsilon.(T)M.sup.b (T).
Typical values of .epsilon.(T) at T=300K for a few solid materials are given in Table 1. Kirchhoff's Law states that an object having the maximum possible absorbance also has the maximum possible emissivity.
TABLE 1 ______________________________________ Material Temperature, .degree.C. Emissivity, .epsilon. ______________________________________ Brick, common red 20 0.93 Candle soot (carbon) 20 0.95 Graphite, filed surface 20 0.98 Concrete 20 0.92 Glass, polished plate 20 0.94 Magnesium, polished 20 0.07 ______________________________________
An ideal blackbody can be closely approximated by an aperture in an isothermal cavity. A layer of soot on a smooth surface has reasonably high emissivity (.epsilon.=0.95), but is generally considered to be inconvenient to fabricate, and is easily damaged by surface contact.